1. Field of the Invention
The present invention relates to a display apparatus which performs a gradation display by using a bi-stable display device.
2. Related Background Art
Hitherto, a liquid crystal display apparatus has been known which performs a gradation display by using a ferroelectric liquid crystal (FLC) as a bi-stable display device.
An example of the display device of the kind described above is disclosed in Japanese Patent Appln. Laid-Open No. 61-94023. This known display device has a liquid crystal cell composed of a pair of alignment-treated glass substrates which are arranged to oppose each other leaving a gap of 1 to 3 microns therebetween and which are provided on their inner surfaces with transparent electrodes, the gap between the glass substrates being filled with a ferroelectric liquid crystal.
The display device employing a ferroelectric liquid crystal has the following advantages. Firstly, ferroelectric liquid crystal has spontaneous polarization so that a composite force composed of a force given by an external electric field and a force developed as a result of the spontaneous polarization can be used as the switching force. Secondly, since the direction of longer axis of the molecules of the liquid crystal coincides with the direction of the spontaneous polarization, the liquid crystal display device can be switched by the polarity of an external electric field.
In general, chiral smectic liquid crystal (SmC*, SmH*) is used as the ferroelectric liquid crystal. This type of ferroelectric liquid crystal in a bulk state exhibits such an orientation that the longer axes of the liquid crystal molecules are twisted. Such a twisting tendency, however, can be eliminated when the liquid crystal is charged in the gap of 1 to 3 microns in the liquid crystal cell (see P213-234, N. A. Clark et al., MCLC: 1983. Vol. Vol 194).
FIGS. 11A and 11B show a typical known ferroelectric liquid crystal cell having a simple matrix substrate structure.
Typically, a ferroelectric liquid crystal is used with its two stable states set to light-transmitting and light-interrupting states, respectively, so as to perform a binary display, e.g., display of black and white images. The ferroelectric liquid crystal display device, however, can be used for display of multi-level or halftone images. One of the methods for effecting such halftone image display is to create an intermediate light-transmitting state by the control of the ratio between the two stable states within a single pixel. A detailed description will be given of this method which is known as the area modulation method.
FIG. 8 is a schematic illustration of the relationship between the light transmissivity of a ferroelectric liquid crystal device and the amplitude of a switching pulse applied to the device. More specifically, a single shot of pulse of a given polarity was applied to the cell (device) which was initially in a complete light-interrupting (black) state so as to change the light-transmissivity of the cell. The light-transmissivity after the application of the single shot of pulse varies according to the amplitude of the pulse. The light-transmissivity I was plotted as a function of the pulse amplitude V, thus, obtaining the curve shown in FIG. 8. The light-transmissivity of the cell is not changed when the amplitude V of the pulse applied is below the threshold value V.sub.th (V&lt;V.sub.th) so that the state of light transmission 9(b) is the same as that shown in FIG. 9(a) obtained in the state before the application of the pulse. When the pulse amplitude is increased beyond the threshold value (V.sub.th &lt;V&lt;V.sub.sat), portions of the liquid crystal in the pixel are switched to the other stable state, i.e., to the light-transmitting state, as shown in FIG. 9(c), so that the pixel exhibits an intermediate level of light transmission. As the pulse amplitude is further increased to exceed the threshold level (V.sub.sat &lt;V), the entire portion of the pixel is switched to light-transmitting state, thus achieving a constant light transmissivity.
According to the area modulation method, it is thus possible to display halftone image by controlling the amplitude of the pulse V within the range expressed by V.sub.th &lt;V&lt;V.sub.sat.
A stable analog gradation display could be performed despite any variation in the threshold characteristics in the display area due to variation in temperature or cell thickness, by using the described area modulation method in combination with a driving method which is disclosed, for example, in the specification of Japanese Patent Application No. 3-73127 of the same applicant. This driving method will be referred to as "driving method of prior application" hereinafter.
The driving method of the prior application, however, essentially requires that four writing pulses and auxiliary pulses assisting these writing pulses are used for each pixel, in order to compensate for any fluctuation in the threshold characteristics in the display area. Consequently, an impractically long time, which is about 10 times as long as that required for conventional monochromatic binary display, is required for writing information in the display area.